SOLID-STATE NMR EXAMINATION OF THE FORMATION OF BETA-SIALON BY CARBOTHERMAL REDUCTION AND NITRIDATION OF HALLOYSITE CLAY

Beta-Sialon (Si6-zAlzOzN8-z; 0<z<4.2] can be readily formed from hallo ysite clay [Al2Si2O5(OH)4.2H2O] at 1400-degrees-C by carbothermal redu ction using brown coal and nitridation. A combination of X-ray diffrac tion, transmission electron microscopy and Si-29 and Al-27 solid-state NMR are used to establish the details of the intermediate stages of t he reaction. NMR clearly shows a distinct sequence of reactions of sil ica and mullite formed from the thermal decomposition of halloysite wi th carbon. The carbon initially reacts with free silica and then remov es silicon from the mullite, to form a beta-SiC-like phase, leaving an alumina-rich mullite framework. SiC subsequently reacts with the rema ining silicon from the mullite phase to form some Si3N4 which then com bines with silicon from SiC, the alumina-rich mullite and nitrogen to form beta-sialon. The Si-29 and Al-27 NMR spectra from sialon X-phase are reported.